DETERMINATION OF THE FREE ACID IN SUPERPHOSHATES. 



71 



O. 2 grammes phosphoric acid was exhausted with ether 

 in a Soxhlet extractor for 10 hours. It dissolved com- 

 pletely. The ethereal solution was converted into an 

 aqueous one as with B, and portion containing *2 grammes 

 H 3 P0 4 were titrated against N/10 NaOH, using different 

 indicators. 



The results obtained were as follows: 



— 





Indicator. 



A 



H 3 P0 4 



dissolved 

 in water. 



B 



H 3 P0 4 



in ether 

 (cold). 



O 



H3PO4 



extracted 

 with ether 



Methyl oraDge 



Phenolphthalein 



Cochineal 



68*71 



68*57 



68*41 



68*69 

 61*42 

 70*02 



68-99 

 68-72 

 70*09 



Litmus 



75*12 



74*56 



71*09 



Congo Red ... 

 Lacmoid 



69*45 



70*25 



71*89 

 72*16 



71*00 

 72*82 



Resorein derivative 

 Turmeric 



... 69-10 

 69-87 



72*16 

 71*63 



72*82 

 70-09 



Sodium alizarin sulj 



)honate 69*45 



69*75 



69-17 



Gravimetrically 





69*52 





With regard to the sharpness and delicacy of the end 

 reactions the following notes are appended, from which it 

 will be seen that both for the sharpness of the end reaction 

 and the accuracy of the results the sodium salt of alizarin 

 sulphonate is by far the most satisfactory indicator. 



End points of titrating H 3 P0 4 with NaOH, 



1. Methyl orange — Fairly sharp. 



2. Phenolphthalein — The point at whichijust a trace of pink 



appears is sharp, but after that point each drop up to 4 

 or 5 does not seem to deepen the colour so much as 

 the 6th, 7th, and 8th drop does. 



3. Cochineal — Fairly sharp, the change from red to purple. 



4. Litmus — To first change, namely red to purple, sharp, 



but the purple continues a long time and takes a long 

 time from purple to blue ; unsatisfactory. 



